Bushings are often installed in a particular pattern within a plate for guiding tool bits into a work piece in the required pattern. In production methods, it is necessary from time to time to replace the installed bushings as a result of normal wear and tear. This is most efficiently done by recreating the pattern of bushings within a production drill plate from a master hole pattern plate. In order to do this, the production plate is placed upon the master plate; and the pattern of holes in the master plate is replicated in the production plate.
If the production plate is to include a large number of bushings, it would be difficult to accurately transfer aligned holes from the master plate if one were to simply drill through each hole of the master plate and production plate. Accordingly, in the past the bushing holes of the production plate have been drilled oversize relative to the outer diameter of the bushing. An alignment pin is then pushed through a hole in the master plate and made to pass through the interior bore of an installed production plate bushing to ensure alignment of the bushing bore with the respective hole in the master plate through which the alignment pin passes. The oversize hole provides some tolerance to achieve good alignment therebetween.
In order to secure the bushing within the production plate, castable material such as epoxy has been used between the exterior surface of the bushing and the wall of the bushing-receiving hole. Typically, the exterior surface of the bushing is knurled so as to maximize the interface surface area between the bushing and castable material.
This procedure for forming the production drill plate suffers a major disadvantage in that the bushing is limited in the load it can assume from an inserted tool to the extent of the shear strength of the castable material. This is due to the fact that the outer diameter of the bushing is less than the diameter of its receiving hole in the production plate and the gap therebetween is filled with the castable material When axial loads exceed the shear strength of the castable material, the castable material will fail and the bushing will be torn from the production plate.